This invention relates generally to an electrophotographic printing machine, and more particularly concerns an apparatus for removing magnetic carrier beads and similar particles from a photoconductive surface.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
The electrostatically attractable developing material commonly used in developing systems comprises a pigmented resinous powder referred to here as a "toner" and a "carrier" of larger granular carrier beads formed with steel cores coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the toner powder and the granular carrier. The toner is attracted to the electrostatic latent image from carrier bristles to produce a visible powder image on an insulating surface of the photoconductive material. Generally, in an endless belt printing machine configuration which employs a plurality of magnetic brushes, the brushes are arranged for developing purposes with a run of the belt in the planar orientation.
In most copiers, however, some carrier beads will adhere to the photoconductive surface of the belt after the latter leaves the development zone. These adhering carrier beads prevent intimate contact between the support surface (e.g., a sheet of paper) and the toner particles, and they may affect the quality of the copy produced. In addition, because such adhering carrier beads are hard, they may abrade the photoconductive surface of the belt if not removed prior to reaching the cleaning zone. Consequently, it is highly desirable that all such carrier beads be removed from the belt after the latter leaves the developing zone. It is also desirable that the means used to remove such carrier beads be capable of being easily removed and replaced for servicing, etc., without contacting the surface of the belt in so doing.
A known bead pick-off device, such as that disclosed in U.S. Pat. No. 3,834,804 issued to Bhagat et al., includes a non-magnetic cylindrical pick-off roller rotatably mounted immediately adjacent to the moving photoconductive belt. A stationarily mounted magnet and pole piece located within the roller creates a magnetic field necessary for attracting carrier beads to and holding them on the roller so as to be conveyed along a course away from the photoconductive belt. The magnetic field decreases sufficiently at a location along the course to permit the carrier beads at that location to descend via gravity into a receptacle located beneath the pick-off roller. It is important that the width of the gap between the photoconductive belt and the pick-off roller be maintained with extreme accuracy so that the proper value of the magnetic field is generated; any change in the width of the gap will result in a change in the value of the magnetic field at the site of the carrier beads, producing a field that is either too strong or too weak to properly remove the beads from the photoconductive belt. One limitation of known bead pick-off devices is that the width of the gap cannot be maintained to within the necessary tolerance over a period of time during which the belt is removed and replaced, without the need for also adjusting the gap width.